This invention relates to tools whose application requires that they exhibit a high degree of resistance both to wear and to shock loading. Such tools include many types of cutting, drilling, boring, scraping, and excavating tools, such as sawblades, scraper blades, router bits, drill bits, boring bits and excavating blades and teeth.
More particularly, the invention relates to improvements in the attachment of carbide-containing metal working parts of high wear resistance to the less wear-resistant base metal of the tool. The improved attachment is of such a strong and tough character as to prevent separation of the carbide-containing working part from the base metal of the tool even under conditions of high shock loading and high working temperature which have caused such separation in prior tools. No significant degree of wear resistance of the working part is sacrificed as a result of the improvement, and the cost of manufacture of the tool is reduced considerably thereby.
It has long been known that high wear resistance of working parts such as sawblade teeth, excavating teeth, boring teeth and the like, can be obtained by forming such parts with a relatively high proportion of cemented carbide particles therein. For example Owen U.S. Pat. No. 2,833,638, and the improvements thereto suggested by Jackson et al. U.S. Pat. No. 3,882,594, disclose methods of forming such working parts from cemented carbide particles and an appropriate low-melting matrix metal, typically high in copper content, by placing a quantity of the cemented carbide particles in a refractory mold, pouring molten matrix metal around the particles and subsequently cooling the matrix metal in the mold to achieve a working part of desired shape. The matrix metal must be of a sufficiently low-melting type to avoid melting of the carbide cementing material (usually cobalt, iron or nickel) during molding and during the subsequent attachment of the working part to the base metal of the tool. Unfortunately, the temperature limitations imposed by the presence of the cemented carbide make it extremely difficult to obtain a sufficiently shock-resistant attachment between the working part and the base metal of the tool. The low-melting matrix metal by which the working part is attached to the base metal of the tool, and the necessarily brazed or soldered joint between the two, are of insufficient toughness to reliably prevent the eventual separation of at least some of the wear-resistant parts from the remainder of the tool under the high-impact and high-temperature operating conditions usually encountered in the application of such tools.
A slightly different, but comparable, use of cemented carbide working parts is exemplified by the tools and methods of manufacture shown in Kolesh U.S. Pat. Nos. 2,880,768 and 3,104,562, Anderson U.S. Pat. No. 3,034,378, Kolb U.S. Pat. No. 3,295,396, Sawamura et al. U.S. Pat. No. 3,718,799, and Funakubo U.S. Pat. No. 3,800,633. In all of these disclosures, working parts of cemented carbide preformed by economical powdered metallurgy processes are soldered or brazed to a less wear-resistant base metal of a tool, or are attached thereto by the type of "welding" wherein only the base metal of the tool undergoes any melting, the cemented carbide being brought only to its plastic deformation or forging temperature before the two parts are pressed together because of the above-described temperature limitations imposed by the cemented carbide. This likewise creates an insufficient bond, between the wear-resistant working part and the base metal of the tool, to reliably prevent separation under expected high-impact and high-temperature working conditions.
The above-described problem of separation of cemented carbide working parts from the remainder of the tool is recognized particularly in Connoy U.S. Pat. No. 3,063,310 and to a lesser extent in Replogle U.S. Pat. No. 2,683,923 and Rayniak et al. U.S. Pat. No. 3,241,228. To solve this problem, these patents employ true fusion-type welding processes (i.e. where both the working part and the base metal of the tool undergo melting) to attach the working part to the tool. However, in order to enable the employment of such fusion-type welding, such systems abandon the extraordinary wear resistance afforded by carbide particles and also abandon the economical use of powdered metallurgy as a means to preform the working part accurately and completely prior to its attachment to the base metal of the tool. Accordingly, not only is wear resistance of the working part sacrificed, but once the working part is attached to the tool it must then be shaped by appropriate cutting or grinding processes, thereby also adding greatly to the cost of manufacture of the tool.
What is needed, therefore, is a tool and method of manufacture thereof in which the use of carbide particles in working parts for maximum wear resistance, and the use of economical powdered metallurgy techniques to preform the working part prior to its attachment in order to minimize manufacturing cost, are somehow made compatible with the employment of a highly shock-resistant, fusion-type steel weld attachment of the working part to the base metal of the tool (i.e. a weld wherein two steel surfaces are melted and diffused into each other).